After filing suit last year against several researchers and an academic journal for publishing a paper (the “C17 paper”) that disagreed with his own, engineering professor Mark Z. Jacobson has dropped his suit. In an FAQ document posted on his website, Jacobson defended his original claim, saying that he did not file the suit simply because he disagreed with the scientific conclusions in the C17 paper, but because the paper contained false factual statements upon which its conclusion relied.

Because the lawsuit has been dismissed, no court has weighed in on Jacobson’s claims. But it seems unlikely that his suit would have prevailed because of the scientific nature of the dispute. Defamation suits require a false and defamatory statement of fact, not opinion. Arguably, the statements in the C17 paper were neither fact nor defamatory. Because the scientific method aims to find truth by testing hypotheses, it is unlikely that most scientific papers will contain “the whole truth.” Indeed, scientific history shows that many of our current facts (the earth is round; human flight is possible) were once ridiculed and thought false!

Defamation suits require a false and defamatory statement of fact, not opinion (in addition to some requirements about public figures and matters of public importance, which can be set aside for now). The difference between fact and opinion can get a little murky, especially in a defamation lawsuit.

To illustrate, consider Galileo and some possible defamation claims. If Galileo says, “The sun is the center of the universe,” and the academy responds, “Heliocentrism is foolish and absurd,” that is just an opinion, and Galileo cannot sue for defamation. Generally speaking, adjectives are not actionable. If the academy says, “Heliocentrism is foolish, absurd, and false, because the earth is the center of the universe,” it has included a fact at the end, but it is still an argument, not an actionable fact. Galileo’s statement has to play by the same rules: If he says, “The academy is wrong, because the sun is the center of the universe,” he has arguably stated two facts, but neither is actionable. They are arguments, not defamatory statements. An exchange of ideas—even if they are mutually exclusive—is not actionable.

Defamation protects reputations, not ideas. So, these statements are not actionable because they are discussing heliocentrism, which cannot sue on its own behalf. So, what if the academy says, “Galileo is foolish and absurd”? It is defamatory, but still an opinion. If the academy says, “Galileo thinks the sun is the center of the universe, and so he is foolish and absurd,” there is still no defamation. The first part of the sentence is true: Galileo does think the sun is the center of the universe. The second part of the sentence is still an opinion, even though it says “is,” not “I think”: Galileo may or may not be foolish and absurd.

An actionable statement would need to be closer to this: “Galileo lied about his findings, plagiarized his data and used the findings to swindle his investors.” Those statements refer to past actions, so they are testable facts, and they are defamatory toward Galileo’s character and reputation. The sentence implies, “Don’t trust Galileo!” not because he is wrong, but because he is a liar. That harms his reputation, rather than disagreeing with his conclusions.

While it is possible that a scientific claim could be the basis of a defamation lawsuit, it is unlikely that it would succeed. Arguments do not usually damage reputations and are usually made in the public square, not before a court.